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Low Mass X-ray Binary 4U1705-44 Exiting an Extended High X-ray State

ATel #10742; Rebecca Phillipson (Drexel University), Patricia T. Boyd (NASA/GSFC), Alan P. Smale (NASA/GSFC)
on 13 Sep 2017; 18:32 UT
Credential Certification: Alan Smale (alan.smale@nasa.gov)

Subjects: X-ray, Binary, Neutron Star, Star

The neutron-star low-mass X-ray binary 4U1705-44, which exhibited high amplitude long-term X-ray variability on the order of hundreds of days during the ~16-year continuous monitoring by the RXTE ASM (1995-2012), entered an anomalously long high state in July 2012 as observed by MAXI (2009-present). It remained high, averaging ~300 mCrab compared to <100 mCrab in the low state, and relatively stable for ~1760 days (> 4 years), from July 2012 through May 2017. This is a substantial deviation from its previous behavior. The previous two high-to-low state transitions occurred with a slope of -20+/-10 mCrab/day in the 2-20 keV bandpass. 4U1705-44 exited its extended high state with a slope of -30+/-10 mCrab/day, marginally faster than the previous two transitions. All three high states sustained average brightness above 200 mCrab. On exiting the anomalous high state, 4U1705-44 entered a low state for ~60 days from May 2017 through July 2017, reaching counts less than 60 mCrab/day. Further analysis is required to determine the change in hardness ratio through the state transitions. Phillipson et al. 2017 (MNRAS, submitted) combined the RXTE and MAXI monitoring campaigns to create a continuous, uninterrupted light curve of the 2-20 keV X-ray flux, nearly 20 years in length, and thus containing many cycles of the longest trends in variability. Phillipson et al. 2017 computed all the zero-crossings and time spent in the low and high states in this extended light curve and found that the source has traditionally transitioned between low- and high-flux states on timescales of hundreds of days or less, with the longest previous excursion in a high state lasting ~280 days (May 1997 through February 1998). 4U1705-44 currently appears to be approaching another high state, but continued monitoring is required in order to determine whether the source will fully revert to its previous pattern of variability, which also included X-ray bursts occurring preferentially when the source was at a low flux level.